1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as, for example, a band pass filter. More specifically, the present invention relates to a structure of a surface acoustic wave filter (SAW filter), in which a plurality of one-port surface acoustic wave resonators (SAW resonators) are connected so as to define a ladder circuit, and a method of manufacturing thereof.
2. Description of the Background Art
Conventionally, a SAW filter has been widely used as a band pass filter. For example, in Japanese Examined Patent Publication No. 56-19765, there is provided a SAW filter having an arrangement such that a plurality of one-port SAW resonators are arranged to constitute a ladder circuit.
Referring to FIGS. 11 and 12, a description of the above-mentioned SAW filter having a ladder circuit structure will be provided. In the SAW filter of Figs. 11 and 12, a series arm for connecting an input end and an output end, and a parallel arm for connecting the series arm and a reference potential are provided. A one-port SAW resonator S1 defining a series-arm resonator is connected to the series arm, and a one-port SAW resonator P1 defining a parallel-arm resonator is connected to the parallel arm. In FIG. 11, only one series-arm resonator and one parallel-arm resonator are shown. However, the number of series-arm resonators and, parallel-arm resonators included in the filter is determined by the desired filter characteristics.
Referring to FIG. 12, the conventional one-port SAW resonator has an electrode structure such that an IDT 51 has a reflector 52 on a first side thereof and a reflector 53 on a second side thereof, all arranged on a piezoelectric substrate (not shown).
The IDT 51 has a pair of bus bars 54 and 55 which extend along a direction in which a surface acoustic wave propagates. The bus bar 54 is connected to one end of each of a plurality of electrode fingers 56. The electrode fingers 56 extend in a direction that is perpendicular to the direction in which a surface acoustic wave propagates, in other words, towards the bus bar 55 on the opposite. side of the bus bar 54. Similarly, the bus bar 55 is connected to one end of each of a plurality of electrode fingers 57. The electrode fingers 57 extend towards the bus bar 54. The electrode fingers 56 and 57 are arranged to be interdigitated with each other.
A plurality of the above one-port SAW resonators are arranged to constitute the ladder circuit as shown in FIG. 11, so as to define a SAW filter. FIG. 13 shows the attenuation-frequency characteristics of the SAW filter.
Since the SAW filter having the ladder circuit structure exhibits small loss and has a wide pass band, SAW filters have been widely used as band pass filters in cellular phones or other similar devices.
However, in recent years, cellular phones have been using a system in which a transmission-side frequency band and a reception-side frequency band are close to each other. Thus, band pass filters should now be able to increase the steepness of the filter characteristics when pass bands are close to each other.
Therefore, in order to increase the steepness of the filter characteristics, in Japanese Unexamined Patent Publication No. 9-167937, there is provided a SAW filter having a circuit structure as shown in FIG. 14. In this case, one-port SAW resonators S1 and S2 are connected to a series arm such that the SAW resonator S1 is connected in parallel to a capacitor 58. According to this conventional device, the addition of the capacitor 58 permits the anti-resonance frequency of the SAW resonator S1 to be decreased so that the steepness of the filter characteristics on the high-frequency side of the pass band can be increased.
However, since the method described in Japanese Unexamined Patent Publication No. 9-167937 requires the addition of the capacitor 58, the size of the SAW filter is increased, and thus, the conventional design is difficult to use if miniaturization of cellular phones or other similar devices is desired. Additionally, since more capacitance is added due to the capacitor 58 to the SAW resonator S1, the attenuation within frequency bands other than the pass band is reduced.
Note that in a SAW filter having the ladder circuit structure in which frequency bands are spaced from the pass band, attenuation is determined by the capacitance ratio of a parallel-arm resonator and a series-arm resonator. In general, when the capacitance of the series-arm resonator is increased, the attenuation is decreased. Consequently, as described above with respect to the conventional device, when the series-arm resonator S1 is connected in parallel to the capacitor 58, the attenuation outside of the pass band is deteriorated, as in the case in which the capacitance of the series-arm resonator is increased.
In order to prevent the above-described decrease in the. attenuation within the frequency bands which are spaced away from the pass band, a solution is to reduce the capacitance of the electrode of the series-arm resonator. However, when the capacitance of the electrode of the series-arm resonator is reduced, it is necessary to reduce the number of pairs of electrode fingers and the interdigitating width thereof, which leads to difficulties in obtaining desirable resonance characteristics.
To overcome the problems described above, preferred embodiments of the present invention provide a SAW filter that increases the steepness of the filter characteristics on the high-frequency side of a pass band while facilitating miniaturization of the filter, suppressing a decrease in the attenuation within frequency bands spaced away from the pass band, and achieving excellent resonance characteristics.
According to a preferred embodiment of the present invention a SAW filter includes a plurality of interdigital transducers arranged on the piezoelectric substrate so as to define a plurality of one-port SAW resonators, the plurality of one-port SAW resonators being connected so as to constitute a ladder circuit having a series arm and a parallel arm. Each of the plurality of the one port SAW resonators includes first and second comb electrodes. The first and second comb-shaped electrodes, each of which has a plurality of electrode fingers and a bus bar connected to first ends of the plurality of electrode fingers, are interdigitated with each other so that second ends of the plurality of electrode fingers of each of the first and second comb-shaped electrodes extend toward the bus bar of the other comb-shaped electrode to define the interdigital transducer. In the interdigital transducer of at least one of the plurality of one-port resonators, a gap between the bus bar of the first comb-shaped electrode and the second ends of the electrode fingers connected to the bus bar of the second comb-shaped electrode is within a range of about 0.50 xcex to about 4 xcex, where xcex is a wavelength of a surface acoustic wave to be excited on the piezoelectric substrate.
In another preferred embodiment of the present invention, at least one one-port SAW resonator is preferably connected to the series arm of the ladder circuit. In this case, a ripple which is close to the pass band on the high-frequency side of the pass band is moved such that the steepness of the filter characteristics on the high-frequency side of the pass band is increased via the attenuation of the ripple.
In another preferred embodiment of the invention, the ladder circuit may have a plurality of series arms, and the gap as described above in the interdigital transducers on all of the series arms may preferably be set within the range of about 0.50 xcex to about 4 xcex. In this case, a synergistic effect of increasing the attenuation due to the ripple in each of the one-port SAW resonators connected at the series arms is achieved. Thus, excellent SAW filter characteristics including steeper filter characteristics at the high-frequency side of the pass band are achieved.
In another preferred embodiment of the present invention, if the one-port SAW resonator uses a LiTaO3 substrate as the piezoelectric substrate, the above-mentioned ripple clearly appears between the resonance frequency and the anti-resonance frequency. Thus, the ripple can be effectively used to increase the attenuation so that the steepness of the filter characteristics when close to the pass band are greatly and efficiently increased.
In another preferred embodiment of the present invention, a method of manufacturing a SAW filter includes the steps of providing a piezoelectric substrate, forming a plurality of interdigital transducers on the piezoelectric substrate so as to form a plurality of one-port SAW resonators, each of the plurality of one port SAW resonators including first and second comb electrodes each including electrode fingers and a bus bar, wherein the plurality of one-port SAW resonators are connected so as to constitute a ladder circuit having a series arm and a parallel arm, and creating a gap between the bus bar of the first comb electrode and ends of the electrode fingers of the second comb electrode that is within a range of about 0.50 xcex to about 4 xcex within at least one of the interdigital transducers such that a ripple occurs within a pass band of the surface acoustic wave filter, where xcex is a wavelength of a surface acoustic wave to be excited on the piezoelectric substrate.
Other features, elements and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.